Method for casting readily oxidizable metals such as magnesium and alloys thereof



i No Drawing Patented Feb. 21, 1933 UNITED STATES PATENT OFFICE JO N A sum JAMEs B. EEID, AND HANs A. REIMERS, F MIDLAND, MICHIGAN, As- SIGNORS 'ro 'rHE Dow CHEMICAL coMrANY,-oE MIDLAND, M CHIGAN, A CORPO- RATION OF MICHIGAN METHOD Eo-B CAsI'INe READILY oxIDIzAELE METALS sUCH As MAGNESIUM AND ALLoYs 'r EEEoE The present invention relates to methods of casting readily oxidizable metals, particularly magnesium and alloys thereof wherein the magnesium content predominates. One object of the present invention is to per- 1 mit casting such metals in moist or .green sand moldsand the like, whereby the metal is protected, from attack by air or by the steam generated in the mold when the hot metal is poured. Another object of our invention is to produce castings of the aforementioned metals the surface of which has" a good appearance and is substantially free practice due to the great ailinity foroxygen this metal possesses at elevated temperatures. In the molten state at the temperatures employed inl' casting,,the metal is capable of combining spontaneously with oxygen, which may be derived either from the atmosphere or from substances containing oxygen'that are in'contact With/the metal, such as water or steam or other oxvgen compounds in the molding material;

bus it is practically impossible to make a sound casting of magnesium in a conventional type of mold in which the molding material has been rendered moldable with water, inasmuch as the steam generatedfromthe' water in the mold material by the hot metal produces an. atmosphere in which magnesium oxidizes rapidly and may even cause the metal to ignite and burn so long as oxygen oroxygen-co'ntaining substances have access to it. Various casting methods have been developed to overcome the tendency for the water i or other oxygen compounds in the mold to attack the magnesium. Heretofore, for example, magnesium has been cast in water- Application filed Kaye, 1932. Serial No. 809,262.

bound sand molds in which the sand of said molds has been mixed with an oxidation in- -h1b1tor,.suchas sulphur or boric acid, so as they react with sand, forming various compounds therewith so that the mixture of sand with said agents soon loses the property of inhibiting'attackupon the metal by the water in the mold. Furthermore, the admixture of the aforesaid fluorides with molding sand causes the sand to swell and interferes with the forming ofmolds therewith. It has been suggested to allow the sand mixture to stand for a period of time before use in order to permit the swelling to complete itself before the sand molds are made. The tendency for these substances to decompose and lib- I erate ammonia, especially on being heated .during the casting process, also results in rapid impoverishment of the molding mixture with respect to the agent added, thus necessitating frequent re-addition of the same. I These agents in the molding sand also produce on the surface of castings made therein an adherent coating which detracts from the appearance of the casting. Furthermore, while the addition of the proper amount of one of the aforementioned agents to the molding sand prevents'oxidation of a casting of uniform cross section, with Qther types of castipgs having an irregular or non-- uniform cross section it often happens that,

vide'protection of the heavy portions of the casting, the thin sections become pitted; or,

if a smaller quantity of said agents is emitors, have the peculiar. disadvantage in that when sufficient of'the agent is added to prov (2) the formation of pits and blowholes; (3) the formation of superficial coatings or films of a non-metallic nature on the surface of the castings which give them'a rough, discolored or otherwise unattractive appearance.

We have discovered that the aforemen tioned difficulties may be practically overcome by employing certain new and improved agents for incorporating in molding sand, which are more stable and permanent than the compounds aforementioned, do not cause swelling of the molding sand, and do 2'5. may produce castings having a surface finish not produce an unsightl surface on the casting. We have found, or instance, that we which is clean and bright and which is also smooth and free from imperfections, such as pitting and blowholes, when the sand is treated with our new agents.

To the accomplishment of the foregoing and related ends, the invention, then, consists of the methods hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail several forms of molds and methods of carrying out the invention, such disclosed molds and methods illustrating, however, but several of the various ways in which the principle of the invention may be used.

Our invention is based upon the discovery ofthe fact that hydrogen fluoride in aqueous solution, when added to a permeable foundry sand in which enough water is present to render the sand moldable, permits casting magnesium without detrimental attack upon the metal by steam generated in the mold. However, since hydrogen fluoride, when so employed, attacks the sand more or less rapidly, the quantity of the same which is presentas such becomes diminished after a time to a point where the amount may be in adequate to prevent oxidation of the metal. We have discovered that we may overcome this disadvantage by mixing with the molding sand certain metallic salts of hydrogen fluoride which are of low volatility and are not quickly displaced from the sand when in use. The salts which we have found ap plicable in the sand mixture with hydrogen fluoride, or which may be substituted for it, are the simple binary fluorides of aluminum (lilF antimony (SbF calcium, chromlum (CrF iron (FeF lithium, zinc, bismuth, or mixtures of these salts. Also, it

has been discovered that, when employing relatively small amounts of the aforemen tioned metallic fluorides, certain other salts or acid salts may be advantageously employed to replace a part or all of the hydrogen fluoride. For example, potassium acid sulphate (KHSO in conjunction with these metallic fluorides effectively inhibits burning or oxidation. The principle of our invention consists in providing a strongly acidified moist atmosphere in contact with the hot metal within the mold at the time of casting, which atmosphere is developed by the heat of the cast metal and slightly attacks the surface of the casting, and prevents the formation of magnesium oxide films thereon. Thus, by our improved method the surface of the casting presents a clean and bright appearance and is free from pitting. These new agents are much less volatile than ammonium salts and are retained in the sand for long periods of time, thus permitting numerous castings to be made without renewal of the agent.

. Suitable procedures to follow for casting magnesium according to our invention are given in the following examples in which an open or permeable type of foundry sand was employed.

Example 1 To foundry sand was added 2 per cent of hydrogen fluoride and the whole thoroughly mixed and tempered with water. Castings of magnesium alloys poured in molds made of this mixture were free from flaws and possessed a clean surface with metallic luster.

Ewa/mple 2 Foundry molding sand was thoroughly mixed with about 5 per cent of antimony fluoride and tem red with water, then formed into 8. mol Castings obtained from such molds were sound and of clean appearance. The fluorides of chromium, zinc, bismuth, etc., employed in similar manner gave equally good castings.

Example 3 A molding sand was thoroughly mixed with about 5 per cent of finely powdered calcium fluoride and about 3 per cent of potassium acid sulphate and the mixture tempered with water. Castings oured in molds employing this molding mixture were free from flaws.

-Ewample 4 A similar molding sand was thoroughly mixed with about 5 per cent of aluminum fluoride and 2 per cent of hydrogen fluoride, tempered with water and then formed into a mold. The casting obtained from this mold was clean and free from flaws.

The amount of such agents or mixtures thereof which may be emplo ed varies with the type of'castin to be ma e and with the nature of the mol ing material. In making large castings it is preferable to employ a greater proportion of inhibitor than when relatively small castings are made. Normal ly the foundry molding material maybe intimately mixed with from 1 to per cent by Weight of any one of the simple binary metallic fluorides aforementioned, or with hydrogen fluoride in amount from 0.5 to 3.0 per cent. Mixtures of two or more of these substances may be employed, the sum of which need ;not exceed about percnt, with or without up to 6 per cent of potassium acid sulphate. For castings having relatively thin sections, smalleramounts than the maximum stated may be used, preferably from 3 to 7 per cent of at least one of the binary fluorides or from 0.5 to 3.0 per cent of hydro.- gen fluoride, while for medium section castings approximately 5 per cent of the same may be used, with or without up to 6 per cent of potassium acid sulphate. It is to be under stood, however, that the quantity of one or more of such agents employed in a given case is to be governed by the size and character of the casting. When adding hydrogen fluoridle to the molding mixturev it is convenient to use commercial aqueous solutions of the same.

Naturally the effectiveness of any oxida-. tion inhibitor for use as hereinbefore described vai'ies withthe permeability of the sand, being somewhat greater with sands of high permeability thanwith those of relatively lowpermeability. The molding sand should, therefore, be sufficiently open or permeable to permit of free escape of gases out of the mold, which are formed b contact with the hot metal as it is poure into the mold. In general we have found that a permeability factor of 30 or more gives satisfactory results, although in many cases good results may be obtained with sands of somewhat lower permeability. The definition of the term permeability 'as used herein and the method of m'easur'ngits numerical value is described in the ulletin. Testing and GradingFoundry Sands, published by the American Foundrymens Association, .March 7 While we have described our invention more particularly in terms ofmolding sand having in admixture therewith one or more of the agents above specified, our invention is-not necessarily limited thereto, inasmuch as these agents are effective as oxidation inhibitors when used in mold facings or in soluthe mold parts coming in contact with the molten metal maybe dusted or otherwise coated with theaforementioned substances. The

tion sprayed upon the inner surface of the. mold or upon baked cores, or the surface of necessarily limited to molds made from sand since other moldable materials such as graphite, pulverized carbon, coke, and the and a relatively small amount of a substance selected from the group consisting of aluminum fluoride, antimony fluoride, sbr g, calcium fluoride, chromium fluoride CrF iron fluoride (FeF zinc fluoride, bismuth fluoride, hydrogen fluorid forming said mixtureinto a mold, and casting magnesium therein.

2. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and a relatively small amount of at least one of the substances selected from the group consistin of aluminum fluoride, antimony fluoride SbF calcium fluoride, chromium fluoride (CrF iron fluoride (FeF zinc fluoride, bismuth fluoride, hydrogen fluoride, forming said mixture into a mold, and casting magnesium therein.

3. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of "foundry sand and from 1 to 10 per cent of at least one of the substances selected from the groupv consistin of aluminum fluoride, antim'ony fluoride SbF calcium fluoride, chromium fluoride (GrF iron flu6ride (FeF z nc fluoride, bismuth fluoride, hydrogen fluoride, forming said mixture into a mold, and casting magnesium therein.

4:.The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from. 1 to 10 percent of at least one of the substances selected from the group consisting of aluminum fluoride, antimony fluoride (SbF calcium fluoride, chromium fluoride (CrF iron fluoride (FeF zinc fluoride, bismuth fluoride, hydrogen fluoride,

and up to 6 per cent ofpotassiu'm acid sulphate, forming said mixture into a mold,- and casting magnesium therein.

5. The method of casting magnesium and alloys thereof in sandmolds which comprises forming an intimate mixture of foundry sand and from 1 to 10 per cent of at least one.. of the substances selected from the group consisting ofaluminum fluoride, antimony fluoride (SbF calciumfluoride, chromium fluoride (CrF ironfluonde (FeF zinc fluoride, bismuth fluoride, and from 0.5 to 3 per cent of hydrogen fluoride, forming said mixture into a mold, and casting magnesium therein.

6. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 1 to 10 per cent of at least one of the substances selected from the group consisting of aluminum fluoride, antimony fluoride (SbF calcium fluoride, chromium fluoride (CrF iron. fluoride (FeF zinc fluoride, bismuth fluoride, and from 0.5 to 3 per cent of hydrogen fluoride and up to 6v per cent of potassium acid sulphate, forming said mixture into a mold, and casting magnesium therein.

7. The method of casting magnesium and alloys thereof in sand molds which comprises forming. an intimate mixture of foundry sand and from 3 to 7 per cent of at least one of the substances selected from the group consisting of aluminum fluoride, antimony fluoride (SbF calcium fluoride, chromium fluoride (CrF iron fluoride (FeF zinc .fluoride, bismuth fluoride, forming said mix ture into a mold, and casting magnesium therein.

8. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 3 to 7 per cent of at least one of the substances selected from the group consisting of aluminum fluoride, antimony fluoride (SbF calcium fluoride, chromium fluoride (CrF- \iron fluoride (FeF zinc fluoride, bismuth fluoride, hydrogen fluoride, and up to 6 per cent of potassium 'acid sulphate, forming said mixture into a mold, and casting magnesium therein.

9. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 3 to 7 per cent of at least one of the substances selected from the group consisting of aluminum fluoride,,antimony fluoride (SbF calcium fluoride, chromium fluoride (CrF iron fluoride (FeF zinc fluoride, bismuth fluoride, and from 0.5 to 3 per cent of hydrogen fluoride, forming said mixture into a mold, and casting magnesium therein.

10. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 3 to 7 per cent of at least one of the substances selected from the group consisting of aluminum fluoride, antimony fluoride (SbF calcium fluoride, chromium fluoride (CrF iron fluoride (FeF zinc fluoride, bismuth fluoride, and from 0.5 to 3 per cent of hydrogen fluoride and up to 6 per cent of potassium acid sulphate, the total amount of the said substances added to the forming said mixture into a mold, and casting magnesium therein.

11. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 3 to 7 per cent of at least one of the substances selected from the group consisting of aluminum fluoride, antimony fluoride (SbF calcium fluoride, cromium fluoride (CrF iron fluoride (FeF zinc fluoride, bismuth fluoride, and from 0.5 to 3 per cent of hydrogen fluoride and up to 6 per cent of potassium acid sulphate, forming said mixture into a mold, and casting magnesium therein.

12. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 1 to 10 per cent of aluminium fluoride, from 1 to 10 per cent of antimony fluoride, and from 0.5 to 3 per cent of hydrogen fluoride, tempering the mixture with water, forming said mixture into a mold, and casting magnesium therein.

13. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 1 to 10 per cent of aluminium fluoride and from 1 to 10 per cent of antimony fluoride, tempering the mixture with water, forming said mixture into a mold, and casting magnesium therein.

14. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand from 1 to 10 per cent of aluminium fluoride and from-0.5 to 3 per cent of hydrogen fluoride, tempering the mixture with water, forming said mixture into. a mold, and casting magnesium therein.

15. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 1 to 10 per cent of antimony fluoride and from 0.5 to 3 per cent of hydrogen fluoride, tempering the mixture with water, forming said mixture into a mold, and casting magnesium therein.-

16. The method of casting magnesium and alloys thereof insand molds which comprises forming an intimate mixture of foundry sand and from 3 to 7 per cent of aluminum fluoride, tempering the mixture with water, forming said mixture into a. mold, and casting magnesium therein.

17. The method of casting magnesium and alloys thereof in sand molds which comprises forming an intimate mixture of foundry sand and from 3 to 7 per cent of antimony fluoride, tempering the mixture with water, forming said mixture into a mold, and casting magnesium therein.

18. The method of casting magnesium and alloys thereof in temporary molds which 5 sand not materially exceeding 15 per cent," comprises forming an intimate mixture of molding material and a relatively small amount of a metallic fluoride characterized by a melting point lying between approximatel 289 and 843 (1., forming said mixture 1nto a mold, and casting magnesium therein.

19. The method of casting magnesium and alloys thereof in temporary water-bound molds which comprises forming an intimate mixture of molding material and a relatively small amount of one of the fluorides selected from the group consisting of aluminum fluoride, antimony fluoride, chromium fluoride, iron fluoride, bismuthfluoride, forming said mixture intoa mold, and casting magnesiumtherein.

Signed by us this 29 day of April, 1932.

JOHN A. GANN. JAMES B. REID.

HANS A. REIMERS. 

